Instruments capable of continuously indicating the chemical composition of blood have proved to be useful in regulating operative and postoperative managements of patients, and in teaching and research. At first, such instruments were used with sensors mounted directly in the extracorporeal blood circuit that is used for perfusion of open-heart surgery patients. Later, continuous monitoring of both machine and patients was conducted by means of continuous withdrawal of blood pumped into external cuvettes equipped with appropriate sensors. Satisfactory systems are now provided for a rapid and accurate measurement of blood composition such as pH, pCO.sub.2 and pO.sub.2.
In addition to the analytical techniques mentioned above, oxygen and carbon dioxide have been measured on the skin by virtue of their diffusing through it. Recently, the continuous monitoring of blood oxygen by a heated electrode positioned on hyperemic skin has been accomplished. Substances such as halogenated organic compounds, particularly fluorinated compounds, have also been found to diffuse through the skin and have been measured. For instance, with reference to U.S. Pat. No. 3,911,138, quantitative measurements have been made of skin-diffused fluorinated compounds by gas chromatography and electron-capture detectors.
Other techniques have been employed for measuring biological substances in the blood. For instance, ethanol is currently measured in blood, either directly or by a breath sampling, by classical chemical, gas chromatographic and enzyme methods. One of the alcohol enzyme methods depends upon the polarographic measurement of hydrogen peroxide, while others depend upon the consumption of oxygen. However, none of these methods readily lend themselves to continuous monitoring.
In brief, while there are a variety of techniques available for the measurement of blood gases and other substances, new methods are desired which more readily lend themselves to continuous monitoring or enable the measurement of key biological substances.